Controllable pitch propellers are becoming more and more popular in marine vessels. The pitch control gives the operator a chance to alter the speed of the marine vessel by changing the blade angle or pitch of the propeller, and more importantly, to change the direction of movement of the marine vessel by turning the propeller blades from ahead direction to astern direction, whereby there is no need to provide the vessel with such a gearbox that is capable of changing the rotational direction of the propeller, or need to reverse the rotational direction of the engine. Such controllable pitch propellers are arranged in a so-called hub. The marine vessel propulsion arrangement comprises an engine, a drive means, a drive shaft and the hub with the propeller blades. The drive means is normally a reduction gear or an electric drive motor, which is used to drive the drive shaft. The pitch of the propeller is controlled by means for turning the propeller blades. The propeller blade turning means comprise actual mechanical turning arrangement arranged in the hub, and means for actuating the mechanical turning arrangement. The mechanical turning arrangement comprises a crank ring for each propeller blade. The propeller blade is rotatably coupled and sealed to the body or casing of the hub by means of the crank ring. The crank ring has a non-central or non-concentric pin extending towards the inside of the hub. The pin is fitted in a groove provided on a member arranged concentrically in the hub body and moving in the direction of the axis of the propeller. The groove extends, preferably, in a direction perpendicular to the axis of the propeller. Now that the moving member is shifted in axial direction by means of the actuating means, the crank ring/s and the propeller blade/s are forced to turn, whereby the pitch of the propeller blades is controlled.
There are basically two types of actuating means, i.e. a mechanical one and a hydraulic one. The mechanical one comprises a rod extending along a central bore in the drive shaft from the drive means to the inside of the hub such that the rod is coupled inside the hub to the movable member, and at the drive means end of the drive shaft to means for moving the rod axially. With regard to the hydraulic actuating means, of which U.S. Pat. No. 4,028,004 may be presented as an example, the movable member inside the hub is arranged to work as a piston in a hydraulic cylinder. In other words, on both axial sides of the movable member there are chambers, so called astern chamber and ahead chamber, into either one of which, when pitch control is desired, pressurized oil is delivered depending on the direction the propeller blades are to be turned. If the pressurized oil is taken to the ahead chamber, the piston, i.e. the movable member, turns the propeller blades such that the propeller blades move the marine vessel in ahead direction. And if the pressurized oil is taken to the astern chamber, the piston, i.e. the movable member, turns the propeller blades such that the propeller blades move the marine vessel in astern direction. Naturally, in the intermediate positions of the piston, the propulsive force of the propeller is reduced due to reduced pitch of the propeller blades.
The pressurized oil is taken to the astern and ahead oil chambers via a central bore in the drive shaft and by providing the central bore with a concentric tube such that two separate oil passages are formed. The oil is provided in the oil passages by means of an oil distribution box arranged in connection with the drive means, i.e. the purpose of the oil distribution box is to deliver oil from stationary oil pipes to rotary oil passages in the dive shaft. The oil distribution box receives pressurized oil from a so called hydraulic powerpack such that the operator of the marine vessel controls, by means of a pilot-operated main control valve arrangement, into which oil passage the pressurized oil is directed, whereby the other oil passage acts as a return passage returning the oil to the hydraulic powerpack. In other words, the hydraulic powerpack receives oil from an oil tank, pressurizes the oil to a required pressure, and by means of the above mentioned pilot-operated main control valve arrangement delivers the pressurized oil to the desired application.
The propeller pitch control works normally such that when starting to move the marine vessel in ahead direction the operator moves the pilot-operated main control valve in the ahead position whereby pressurized oil is allowed to enter the ahead oil chamber in the hub and the oil in the astern oil chamber is allowed to escape the astern oil chamber so that the movable member may turn the propeller blades in ahead direction against the hydraulic pressure water subjects to the rotary propeller blades. When the propeller blades are in their desired position the pilot-operated main control valve is moved in a position blocking the flow in both oil passages, whereby the blades are maintained in their position by keeping the ahead oil path blocked and the blades creating a certain pressure in the ahead oil path. If no leakage would occur in any part of the piping, the propeller blades would be locked in their current position.
However, keeping the ahead flow path closed by means of the pilot-operated main control valve is not, in practice, possible, as, in the oil distribution box the oil flows from stationary oil pipes to rotary oil pipes, causing a significant leakage due to the nature of this type of seal (annular pressure seal). An option to solve this leakage-related problem would be to move the pilot-operated main control valve to compensate for this leakage, but this would be very energy inefficient and difficult to control. Therefore a blocking valve (pilot-operated non-return valve or similar), not discussed in the above-mentioned U.S. Pat. No. 4,028,004, is placed in the ahead rotary oil pipe. In the ahead rotary pipe because propellers are normally designed such that the blades turn automatically to their astern direction even if no pressure is introduced in the astern oil chamber. Oil flow from the ahead oil chamber to the pilot-operated main control valve is blocked (leakage of the blocking valve is minimal and can be considered insignificant with respect to keeping the propeller blades at their current position) and, in fact, to the oil distribution box, whereby the oil is pressurized between the piston and the blocking valve in the ahead oil path, effectively locking the propeller blades in their current position without any need of supplying additional pressurized oil from the hydraulic powerpack.
The blocking valve functions such that pressurized oil flowing from the pilot-operated main control valve towards the ahead oil chamber may pass the blocking valve, so as to be able to change the blade pitch towards ahead. The blocking valve is connected by means of a pilot line to the astern flow path such that pressurized oil flowing from the pilot-operated main control valve towards the astern oil chamber, when exceeding a certain pressure (determined among others by the pilot ratio of the blocking valve), is able to open the blocking valve thereby allowing oil to escape from the ahead oil chamber and therefore to allow the pitch of the blades to be changed to astern. Allowing pressurized oil to flow towards the astern oil chamber will cause the movable member to eventually be stopped by a mechanical stopper. In other words, when the propeller blades are in their “full astern” position the mechanical stopper will lock the propeller blades in their current position, not the blocked oil in either one of the oil chambers.
In addition to the hydraulic actuating means the hub needs oil for the lubrication of the mechanical turning or control arrangement. There are in fact two options for arranging the lubrication. A first one would be to supply oil to and from the hub through the stern tube. But, however, it is not a preferred option, as it would connect the hub lubrication system with the stern tube lubrication system, whereby a problem, for instance water ingress, in one system would result in the same problem in the other system, too. A second, preferred, option, discussed in the above-mentioned U.S. Pat. No. 4,028,004, too, is to use the central bore of the shaft as the safe route for the lubricant delivery into the hub. Therefore, the central bore of the drive shaft is provided with another concentric tube so that three separate flow passages are arranged within the drive shaft. The two innermost flow passages communicate with the astern and ahead oil chambers whereas the outermost flow passage between the moving member and the body or housing of the hub communicates the lubricant chamber.
The lubricant chamber is provided with oil from the oil tank by gravity. The oil tank is arranged above the waterline of a fully loaded marine vessel for safety purposes such that the hydrostatic pressure inside the hub remains always higher than the water pressure outside the hub, whereby in case of sealing failure in the hub water is not able to enter the lubricant chamber, but the lubricant leaks to the surrounding water.
However, practice has shown that water may enter the inside of the hub, i.e. to the lubricant chamber, by condensation, by leakage, or for some other reason. Thus, it was, at some point of time, considered that some kind of lubricant circulation is needed in place of the earlier practice, discussed in the above-mentioned U.S. Pat. No. 4,028,004, where the same lubricant remained in the lubricant chamber and collected water and any other impurities until specific measures were taken to replace the used lubricant with fresh one.
Naturally, the first suggestion was to arrange a third concentric tube in the central bore of the drive shaft, which would have required also changes in the oil distribution box. As such a construction was considered quite complicated and risky, it was, for the second suggestion, realized that the hub already has pressurized oil in their ahead and astern oil chambers at least while the pitch of the propeller was changed. To take this suggestion into use only meant that an oil flow channel should be arranged from one of the ahead and astern oil chambers to the lubricant chamber. Since such an oil flow channel is a static element without any need for repair or maintenance it was taken into use between the astern oil chamber and the lubricant chamber. The arranging of the lubricant channel to start from the ahead oil chamber is not possible as the ahead oil chamber has to be kept blocked and pressurized when sailing ahead. Naturally, the oil flow channel, as well as the lubricant circulation, was in use only when moving the propeller blades to their astern position, i.e. the oil circulation took place only occasionally for a substantially short period of time. However, at that time it was considered sufficient as the mineral oil that was used for both the hydraulics and lubricating the mechanical turning arrangement was capable of dealing with certain amount of water without damage to either the oil itself or the surfaces it was supposed to lubricate.
However, now the environmental requirements are about to change such that in all positions where there is even a minor risk of leakage of oil to the surrounding water such oil has to be used that degrades easily when in contact with water. This is, naturally, a good property for oil in view of environment, but not so desirable in view of lubrication, as such an environmentally acceptable lubricant has to be watched continuously, as it may lose its lubrication capability in days or weeks depending on the amount of water that has got into contact with the oil. Therefore, modern controllable pitch propellers have to be provided with an oil circulation, at least when sailing ahead, to ensure that water is not able to collect into the hub.